Optical storage can achieve high areal density data storage by using a tightly focused laser beam. Magneto-optical materials, for example, can be used to produce an areal data density of up to or higher than about ten gigabits per square inch. A typical optical storage system usually focuses a monochromatic optical beam to a small spot on a recording layer by using an optical head with a large numerical aperture. The optical head is conventionally positioned over the medium by a spacing greater than one wavelength. This is called a "far-field" optical configuration.
For a given wavelength, the areal data density of an optical storage system can be increased by focusing an optical beam onto a flat surface of a solid transparent material with a high refractive index that is implemented in the optical head. The diffraction-limited focused spot size is hence reduced by a factor of the refractive index compared to the spot size in air.
An optical storage system can also be configured to operate in a "near-field" configuration where the optical head and the optical medium are spaced from each other by a distance on the order of or less than one wavelength. The optical coupling between the optical head and the medium, therefore, can be effected by evanescent optical coupling, which is not possible in a far-field configuration. A numerical aperture of the optical head in such a near-field configuration can be greater than unity. Hence, a near-field optical storage system can be used to achieve a focused beam spot size less than one wavelength and to realize a high areal storage density beyond the capability of many far-field systems. U.S. Pat. No. 5,125,750 to Corle and Kino discloses a near-field optical recording system based on a solid immersion lens.
Whichever optical configurations are implemented, the design and construction of the optical head are often critical to the performance of an optical storage system. Certain aspects of the optical head design and the optical storage system are described by Alan B. Marchant, in "Optical Recording," Addison-Wesley Publishing (1990). In addition to the focusing of a read/write beam, the properties of the optical head may also affect other operations of the system including the signal detection and beam tracking on the data tracks.